Temperature transmitters are vital to many of today’s process control systems. However, not all units are made the same, and ensuring you pick the suitable device for the proper operation can be a complex decision. To understand the important aspects of modern temperature transmitters—reliability, safety, and useability, in particular—Control talked with Greg Pryor, temperature product marketing manager at Endress+Hauser USA.
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Q: How have temperature transmitters evolved, and what is their impact on various industries?
A: At their core, most temperature transmitters convert sensor output to a 4-20 mA analog signal. I'd say the biggest evolution in the last several years is the addition of other communication methods. HART protocol has been around for a long time, and then digital protocols (Profibus, Foundation Fieldbus, etc) began to come into play. In addition, more recently, we see newer technologies like IO-Link and, even now, Ethernet-APL making its way into transmitters. This allows more flexibility for users to gain additional functionality on the back end by seamlessly integrating the incoming transmitter signals into different system architectures. The other big evolution is customers adopting Bluetooth functionality for re-parameterization in the field. Bluetooth was a little slow to be accepted, primarily because of security risk questions, but that technology is safe, reliable and a considerable time-saver now. It makes it so much easier than climbing to the top of a tank, getting on a ladder, or opening an enclosure to hardwire a communicator to a transmitter to change a temperature range, for example.
Q: Why do they require a strong focus on safety?
A: Temperature is the most common process variable measurement in almost every industry, and a considerable part of the reason is safety. So many processes are temperature-range-dependent, and must be controlled to stay within that range. In heavier industries, like chemicals or oil & gas, temperature spikes above the desired range can cause combustion, unwanted reactions or significant safety risks to plant personnel. In the food & beverage industry, almost every food safety process must reach a specific temperature—think pasteurization of milk, CIP or SIP processes, etc. Failure to achieve the proper temperature can cause unsafe food products to be sent out for consumption, and people can get seriously ill. Accurate temperature control is key to keeping everyone safe across industries, so the focus there is a huge priority.
Q: What factors determine a temperature transmitter’s usability?
A: Having the correct configuration for the transmitter is the key to high usability, but specifying the wrong fit can quickly cause usability challenges. For example, maximum and minimum temperature ranges must be programmed correctly, the transmitter needs to be set for the correct input type, and the wiring must be done properly according to the manufacturer’s instructions. If you receive those details right on the front end, then your transmitter performance during use becomes quite simple. Using Bluetooth technology also makes modifying parameters on installed transmitters much simpler in the field.
Q: What makes today’s temperature transmitters more cost-effective than in the past?
A: I think various factors come into play for this question. Most of the major transmitter manufacturers have gotten to a volume efficiency at the larger scale at this point, so the actual cost to build simple transmitters (and the respective purchase price) has gone down as transmitter use has increased. Because their use has become so much more commonplace in temperature assemblies, most facilities are set up for taking 4-20 mA inputs into their control systems. This allows I/O cards to be more universal than thermocouple or RTD-specific, allowing control systems personnel to realize cost savings just on I/O cards alone. Being able to use the same type of I/O card for temperature, pressure, level, etc, allows for adding the same type of input regardless of the variable input—they're all coming in as 4-20 mA. In addition, they can better control the process due to the increased amount of information they can pull from the transmitter. This allows them to realize higher process efficiencies, greater yields and better product quality. All those benefits add to the bottom-line profitability for any customer.
Q: How accurate are today’s units?
A: Today’s technology allows manufacturers to achieve incredible accuracy. Determining accuracy on a transmitter is complex and is based on formulas derived from various factors, and most manufacturers have specific tolerances posted for each transmitter in their portfolio. One thing to always keep in mind, if accuracy is critical in your application, is that the transmitter is only one piece of the temperature path. The sensor, the wiring, and several other factors come into play when you read the output from the transmitter. You need to add the tolerances of all the components along that path together to achieve true accuracy. That's one reason many users take advantage of sensor-transmitter matching, which allows a transmitter to offset to a specific sensor that's been calibrated to determine its Callendar Van-Dusen coefficients. This virtually eliminates the error from the sensor itself, increasing the entire assembly's accuracy.
Q: What advice can you offer for companies purchasing temperature transmitters? What should they understand about today’s temperature transmitters before buying?
A: The best advice I can give a customer is don’t pay more for features you won’t use. There is a wide range of available transmitters, from very basic functionality to every bell and whistle available. If you're using single-element sensors and going one-to-one to transmitters, you don't need a dual-input transmitter. If you don’t use HART, buy a simple 4-20 mA transmitter. If nobody ever looks at the display, and you'll only read the output in the control room, why get a display? Fit for purpose is the key. Whatever your needs are, a transmitter will probably cover it; don't overspend for functionality you don't need. If you aren’t sure what your options are or what functions you would use, just partner with a reliable vendor that will walk you through to help find the right fit for your application.
